I don't think this should be in use, much less in core (sorry).
in reply to package Foo; isa Bar; why not?
To me, it is one of the first "natural" uses of indirect object syntax.
IMHO, there is only one use for indirect object syntaxt, and it is poetry
The only thing I have against it, is that you would need to say "use isa qw(Bar)". It should be possible to do this shorter.
It's too late. As a core language feature, maybe, but it's too late for that. But by way of yucky yucky yuck yuck and not during compiletime? No.